WIM sensors are sensors that are between one and several meters in length that are installed in a roadway in order to measure the dynamic ground reaction forces of vehicles in order to, from this, determine the weight of said vehicles. The aluminium hollow profile into which a measuring element is installed under preload forms a significant element of the sensor. In this arrangement, supports that are provided for this at the top and at the bottom ensure optimum force flow through the measuring element. The hollow profile comprises a tube part that is responsible for preloading and sealing the measuring element. A known hollow profile for such purposes is, for example, described in U.S. Pat. No. 5,501,111, which is hereby incorporated herein in its entirety by this reference for all purposes. This design comprises a tube with a circular or hexagonal outer diameter and an inner hollow space that laterally, transversely to the direction of force transmission, is surrounded by uniform wall thicknesses and in the directions in which force transmission takes place on the inside comprises parallel surfaces for the purpose of clamping measuring elements. Such systems have shown to be less suitable because the force transmission from the outside to the tube can be controlled only inaccurately.
A further known hollow profile for such purposes is, for example, described in U.S. Pat. No. 5,461,924, which is hereby incorporated herein in its entirety for all purposes by this reference. As an expansion when compared to the embodiment according to U.S. Pat. No. 5,501,111 the embodiment according to U.S. Pat. No. 5,461,924 comprises two force-transmission plates between which the tube is affixed. Since these parallel force-transmission plates are a little more than twice as wide as the tube, the inward transmission force is well defined in this arrangement. A plurality of measuring elements that are preloaded by the tube receive the corresponding signals when a vehicle drives over the device.
FIG. 1 shows a cross section of a hollow profile 1 of a weigh in motion (WIM) sensor according to the state of the art. As a rule the width and height of such profiles range from 50 to 70 mm, while their length (not shown in the drawing) typically ranges from 1 m to 4 m. Such hollow profiles can thus be considered to be elongated relative to their cross-sectional dimensions.
The hollow profile 1 comprises two force-transmission plates 2 arranged parallel to each other and a tube 3 arranged between said plates 2. Said tube 3 is integrally formed with the plates 2 and forms a hollow space 5 internally of the walls that define the tube 3. Two supports 6 that are arranged opposite one another are formed inside the hollow space 5, each one extending away from a respective plate 2. A measuring element 7 is shown by a dashed line forming a rectangle between these supports 6. Such a measuring element 7 is used for completing the WIM sensor centrally in the tube 3 under preload between the supports 6.
In this arrangement the measuring element 7 is shown diagrammatically; in reality it comprises a plurality of measuring elements, in particular of piezo-electric measuring elements. These are arranged equidistantly along the entire length of the hollow profile, centred between the supports 6, for example as plates 2 or as plate pairs 2 one on top of the other, as is also described in U.S. Pat. No. 5,461,924.
The tube 3 comprises two tube segments 8, with a wall thickness 9, which are designed to be mirror-symmetrical with respect to each other, which tube segments 8 join the plates 2 together and on the inside adjoin the hollow space 5.
The tube segments 8 are responsible for preloading the measuring element 7. Preloading is achieved by covering the measuring element 7. For this covering to be achievable in the first place in a stable manner, taking into account component tolerances, it needs to be of a certain magnitude. Nonetheless, the preload force must not be excessive. In order to achieve this, the wall thickness 9 of the tube segments 8 should be as thin as possible.
On the other hand, however, during loading of a WIM sensor very considerable mechanical load peaks are experienced, in particular in the transition region 12 of the tube segments 8. In the case of an insufficient wall thickness 9, these load peaks result in plastic deformation. This would result in a change in the measuring characteristics of the sensor, in the formation of cracks, and in failure of the sensor. Since this sensor is permanently installed in the surface of a road, and any replacement is associated with road closure lasting several hours, replacement of a WIM sensor is to be avoided in all circumstances.
Preloading the measuring element generates significant stress in the tubular part. Additional high stress in the tubular part is generated when heavy vehicles drive over the sensor in the installed state. Since this represents a dynamic load, and frequently high load cycles are experienced when vehicles drive over the device, the profile is subjected to permanent damage, in particular in the region of the transition from the tubular part to the force-transmission plates. It has been shown that conventional tubular profiles cannot withstand the operating loads of WIM applications. Since the tubular part forms a force shunt that reduces the sensitivity of the sensor and thus impairs the measuring results, said tubular part should be designed so as to be as thin-walled as possible and it should not be unnecessarily reinforced.